This invention relates in general to an additive for transformer oils and more particularly to such an additive comprising a mixture of a surfactant and a halogenated hydrocarbon that is a liquid at room temperature.
Transformers are used in electricity and special voltage delivery systems. A transformer can be used to step-up or step-down voltage and change the voltage through electromagnetic induction. A typical high-voltage transformer has massive coil windings around a metal core. The coil winding is typically an insulated copper or other low resistivity metal wire and the core preferably comprises a plurality of thin steel laminations stacked side-by-side. Transformer coil windings of this type are known to breakdown after a period of time by oxidation of the coil windings, and by such occurrences as arcing, metal flaking and by the constant heat created by the transformers. This breakdown is greatly reduced by submersing the coil windings in an enclosed bath of oil. Most high-voltage transformers are oil-filled systems.
Transformer oils must, therefore, be able to enhance the performance of the transformer and prolong the useful life of the transformer before breakdown. The extremitus temperature range that transformer oils are subject to is between 0.degree. F. and 350.degree. F. The conventional operating range is between 60.degree. F. and 190.degree. F. Transformer oils have previously consisted of petroleum based naphthenic oils. It has been forecasted that the supply of such naphthenic oil will be depleted by 1990. As an alternative, petroleum based paraffinic oils and vegetable oils can be used as a suitable transformer oil. Paraffinic oils are generally straight carbon chains. Vegetable and paraffinic oils are more economical and ubiquitous, but have inconsistent breakdown characteristics that requires periodic monitoring, while in use in the transformer, as to its continued effectiveness.
Transformer oils do not have an infinite life span. Most transformer oils undergo auto-oxidation, decreases in pH, and other physical and/or chemical changes that ultimately permit the transformer to fail due to the inadequacies of the oil. Thus, transformer oil additives were developed to improve the longevity and characteristics of transformer oils, and the transformers themselves.
In order for a chemical compound to be an effective transformer oil additive, it must be capable of simultaneously raising or stabilizing the resistivity of the final mixture, maintaining and stabilizing the dielectric strength of the final mixture, and retarding the rate of oxidation of the oil. These objects must be met while also imparting heat stability and fire resistance to the final mixture. It is also important that the final mixture have a low level of environmental toxicity.
The primary reason for transformer oil breakdown, and subsequent transformer failure, is oxidation of the oil. When the oil is oxidized, hydrogen atoms are removed or freed from the oil's molecular chain. This causes the formation of a "free radical" or a highly active intermediate that has a tendency to shear itself and other chains into smaller chains. These smaller, sheared chains have a tendency to associate with available oxygen molecules or other radicals to form "hyperoxides." These chains, as hyperoxides, are capable of attacking the remaining chains to create more free radicals. This sequence of events causes the progressive hydrogen removal on the carbon chains of the oil. As a result, the oil which originally was long, organized carbon chains becomes short carbon chains with a reduced capability for heat transfer and a reduced dielectric strength. This breakdown is continual and self-perpetuating in that organic acids, which amplify the breakdown process, are a by-product of the reaction. Therefore, it is important that an additive for transformer oil have characteristics that limit the oxidation process and resulting breakdown of the oil.
Heretofore, the most widely used transformer oil additives were polychlorinated biphenyls (PCBs). PCBs are very effective in improving the longevity of transformers and transformer oils. Typically, a PCB would be added to the transformer oil to a concentration of greater than 1000 parts per million (ppm) of transformer oil.
Although polychlorinated biphenyls impart ideal characteristics to transformer oils, the use of polychlorinated biphenyls has been drastically reduced due to its extraordinary environmental hazard. Federal regulations now require that transformer oils contain less than 500 parts per million (ppm) polychlorinated biphenyls. Such uses of polychlorinated biphenyls in transformers are prohibited if the equipment poses a risk to food or feed. Federal regulations now also prohibit the manufacture of polychlorinated biphenyls.
Non-polychlorinated biphenyl containing transformer oils are now being used, but these oils are dramatically less effective in that they demonstrate poorer electrical resistance, greater flammability, and a higher tendency to oxidize with the resulting effect of compromising the power factor of the transformers. As a result, electric utilities and service organizations incur significant costs in recycling and exchanging non-polychlorinated biphenyl oils and must also frequently test the non-polychlorinated biphenyl containing oils to assure their continued effectiveness in protecting the transformer. A further consequence of using non-polychlorinated biphenyl oils is that more voltage step-down stations are needed where such stations were less necessary when polychlorinated biphenyl containing transformers were used.
It is thus evident that a need exists for a transformer oil additive that protects transformer oils and transformers from frequent breakdown and enhances the transformers capability and longevity, and that also has a relatively low level of toxicity.
A number of different chemicals or compounds have been tried as substitutes for the polychlorinated biphenyl additive for transformer oils. Pyrizolidines, sulphur compounds and other organo-aromatic compounds have been tried. But while these have been shown to decrease the oxidation of oil, they also decrease the oil's electrical resistivity thus making them less useful as a substitute additive.
Alkylbenzenes and other petroleum compounds have been experimented with and used as substitute additives. These compounds do increase electrical resistance of the oils, but they also auto-oxidize and are highly combustible so as to make the oil flash points dangerously low for safe and effective transformer applications. Additionaly, all of the above additives are considered to be relatively toxic.